Economic and Societal Benefits
Where groundwater storage depletion is large, even larger volumes of groundwater have been withdrawn and used. These uses provide water for drinking, for factories, and for agriculture, and therefore have great value. Such beneficial use and value must be carefully weighed when considering the economic and environmental effects and costs of the storage depletion. For example, substantial and continuing groundwater depletion might adversely impact the availability of water for irrigated agricultural productivity (e.g., CAST, 2019). Water managers and policy makers, of course, must also make their decisions in light of the legal framework applicable to their areas – a framework that is highly variable from one political entity to another (even within a single nation).
Water levels always decline in response to pumping. This is normal and not always a problem. However, if water-level declines in an aquifer or in an individual well are substantial, it can have detrimental physical and economic effects. Large water-level declines lead to reduced well yields and increased energy costs because of the greater lift required to move water from the well to the discharge point at or above the land surface. One reason that a greater lift results in a reduced well yield is because most pumps with a fixed power rating and capacity will produce a discharge of water in an inverse relation to the magnitude of the lift. For a flowing artesian well, the reduced water levels (or heads) will reduce the gradient driving the flow to the land surface, and hence reduce or eliminate the flow discharging from the well. In an unconfined aquifer, lowering the water table will also reduce the saturated thickness of the aquifer adjacent to the open or screened interval of the wellbore, and that will consequently reduce the effective transmissivity of the aquifer at the well, which in turn will reduce the well yield. In a confined aquifer, even as the potentiometric head drops, the aquifer will remain fully saturated, except in extreme circumstances. It is possible that the head can decline to a level below the elevation of the top of the aquifer, in which case the confined aquifer will start to drain and dewater. However, this is a rare occurrence.
If the water level in a well drops below the pump intake, the well will “go dry.” This will have economic impacts on the well owner, who then must choose among several costly alternatives, including lowering the pump (if possible), deepening the existing well, drilling a new deeper well, abandoning the well and the former use of its produced water, or purchasing water (or water rights) and conveyance infrastructure for an alternate source of water supply if one is available and such transfers are legally allowable.
Groundwater storage depletion and the concomitant water-level declines can also have some poroelastic effects and consequences. The most common, widespread, and consequential is land subsidence, which can damage infrastructure and is widely recognized to be associated with groundwater use and storage depletion. Examples of affected areas include the Houston, Texas, area, the Central Valley of California, Mexico City, Bangkok, Tokyo, Jakarta, Venice, and other areas including those discussed by Galloway et al. (1999) and Poland (1984). The mechanisms whereby groundwater withdrawals can cause land subsidence are discussed in more detail in other Groundwater Project books. In summary, if the aquifers contain clayey lenses or layers, then lowering the heads can reduce the pore pressure in these materials, which in turn causes them to compress in an inelastic manner as the clay mineral structure itself realigns in a more compact (and irreversible) manner.
Most depleted groundwater ultimately finds its way into the oceans – the ultimate sink. In a sense, groundwater depletion can be viewed as a large-scale, long-term transfer process of water from the continents to the oceans. If the long-term cumulative volume of depleted groundwater is large enough, it can contribute to sea-level rise, and there is good evidence that it has (e.g., Sahagian et al., 1994; Konikow, 2011; Church et al., 2011; Dӧll et al., 2014). The studies indicate that in the first decade of the 21st century, global groundwater depletion may have contributed 0.3 to 0.4 mm/yr to sea-level rise – about 10 percent of the observed sea-level rise.